THE IMPERFECT LINK BETWEEN VOLCANISM AND MASS EXTINCTION

The suggestion that all post-Palaeozoic extinction events coincide with the eruption of vast provinces of continental flood basalts was first made over 12 years ago, a link confirmed by subsequent improvements in radiometric dating. However, the relationship is by no means clear cut. Thus, the eruptions of two of the largest provinces, the Parana and North Atlantic igneous provinces, do not coincide with mass extinctions, whilst the relatively small Emeishan flood basalts coincide with the major end-Guadalupian extinction. Rapid global warming and the development of marine anoxia are the environmental changes most frequently observed to coincide with mass extinctions, and these events appear to be the proximate cause of several biotic crises (e.g. end-Permian). This implies that the warming effect of volcanic CO2 release is the most consequential environmental factor of flood basalt volcanism. The most intriguing observation is that the "killing efficiency" of flood basalt eruptions declines rapidly after the Jurassic (if the Deccan Traps/K-T mass extinction link is disregarded in favour of a more compelling link with bolide impact). This secular decline in the impact of volcanism is well illustrated by comparison of the end-Permian and late Palaeocene intervals. In the former case the onset of Siberian Traps eruptions coincides with a mass extinction, global warming and a negative C isotope excursion. Faunal radiation did not begin until at least 5 myr after this extinction and the recovery of the C isotope curve was similarly prolonged. In contrast the late Palaeocene event witnessed a similar onset (rapid negative C isotope excursion, global warming, major volcanism), but there was no associated mass extinction and a return to pre-event conditions within a few 10s kyrs. Dramatic improvements in the efficiency of removal of atmospheric CO2 in the past 100 myr, perhaps due to the rise of calcareous plankton, may have ameliorated the effects of more recent large volcanic events.